Cellular base stations containing radio antennas, transceivers and signal processors have long been the backbone of mobile communications, ever since the first commercial cellular networks went live in the early 1980s. Used to transmit and process signals from mobile devices and connect them with the phone network, base stations and the underlying cellular architecture have performed relatively well until now; managing to keep up with the boom in both the number and speed of mobile devices that has occurred with the rollout of third-generation (3G) mobile services.

But there are signs that the current architecture may not be able to cope as well as mobile communications move towards 4G and users demand the kinds of broadband speeds already offered by fixed-line communications. 4G promises data rates of up to 1 gigabit per second (Gbps) for stationary users and up to 100 megabits per second (Mbps) for mobile devices - dozens of times faster than current 3G services.

Several challenges will need to be overcome if market demand for mobile broadband is to be met. To begin with, base stations need to handle an increased range of signals from different communications technologies in order to provide a wider variety of services - from voice and data to streaming video and online games. That has meant the signal processing technology inside base stations has become increasingly complex and will be even more complex in the future.

In addition, 4G will operate at higher radio frequencies in order to provide more bandwidth, meaning the signal cannot travel as far. Therefore, the area covered by a single base station - known as a cell - will be much smaller. More cells and hence more base stations will be needed to provide coverage. But, because interference between base stations remains longer in range than the cell size, system capacity will not increase linearly, and more antennas will also be needed to cover a given area.

In fact, the current cellular architecture could continue to be used in the 4G era if adapted to more and smaller-sized cells, more antennas and base stations with more complex signal processing components. After all, despite the large infrastructure cost, that was the strategy operators followed as they transitioned from 1G to 2G and on to the current 3G. But there may be a better way.

'We tried to find the simplest solution possible and that meant separating the signal processing from the base stations and doing it in a central location,' explains Paulo Pereira Monteiro at Nokia Siemens Networks Portugal.

Bye-bye to base stations?

Working in the EU-funded FUTON project, Mr Monteiro coordinated a team of researchers in the development of a hybrid architecture using high-speed fibre optic cables to transmit signals between 'Remote antenna units' (RAUs) and a central unit where signal processing is carried out. RAUs could therefore effectively replace base stations, providing radio coverage in each cell and converting the radio signals into optical signals to be processed at the central unit.

'RAUs are smaller, cheaper and simpler to install and maintain than full-fledged base stations - they could be installed on a lamppost. Their environmental impact is therefore much less, so too is their energy consumption,' the FUTON coordinator notes.

The RAUs effectively operate as the distributed antennas of a composite base station with the central unit at its core. Fibre-optic cable is the 'logical choice' to transparently transmit information between the central unit and the RAUs, Mr Monteiro says, because of low attenuation and enormous bandwidth. Though radio-over-fibre is not new - it has long been used to transmit cable television signals, for example - the FUTON approach is a fundamentally new implementation of the technology.

Because it enables signal processing from multiple RAUs to be carried out jointly, mobile devices would be able to communicate simultaneously with several antennas with perfect cooperation between them. Currently, signals from different base stations cause negative interference to your signal, but the ability to link with multiple RAUs would provide more bandwidth to more people in their area of coverage.

'Currently, a lot of effort goes into keeping interference between cells to acceptable levels and it is becoming more and more complex as the number of heterogeneous services with different requirements increases, often [needing] a significant re-planning every time market demand requires a network augmentation,' Mr Monteiro says. 'There aren't the same constraints with the FUTON architecture and if the network has to be augmented, the inclusion of new RAUs can be done dynamically.'

The FUTON team showcased their work in a series of proof-of-concept demonstrations that trialled how the system would function in a real-world setting. Mr Monteiro notes, however, that more work needs to be done and consensus among operators, service providers and equipment manufacturers needs to be reached before the architecture is likely to be implemented commercially.

'There are many variables. This is not the only option out there to solve the challenges of providing real broadband speeds to mobile devices but to us it seems to be the simplest and most efficient one,' the FUTON coordinator says.

With that goal in mind, the project consortium, which includes major industrial partners and operators such as Nokia Siemens Networks, Alcatel-Thales III-V Labs, Portugal Telecom, Hellenic Telecommunications and VIVO, are active in standardisation efforts. However, Mr Monteiro says it will probably be 6 to 10 years before the results of the FUTON research feed into commercial applications.

Nonetheless, he notes that the FUTON architecture could help create a whole new segment of the mobile network market. Whereas base stations and network infrastructure have until now been largely the domain of the major mobile operators, Mr Monteiro foresees the potential for third-party entrants to build the radio-fibre infrastructure of RAUs and central units and then rent this to operators - opening the door for new service providers to enter the cellular telecommunications market.

The FUTON project received EUR 6.58 million in research funding under the EU's Seventh Framework Programme (FP7), sub-programme 'The network of the future'.